<!DOCTYPE html>
<html lang="zh">
<head>
    <meta charset="utf-8">
    <title>Responsive Image</title>
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <style>
        .box {
            margin: 30px auto;
            width: 100px;
            height: 100px;
            background-color: blue;
        }

        /* 任务1、使用CSS hacks实现只有在Firefox浏览器下, 且Firefox版本>=2时, box背景为红色 */

    </style>
</head>
<body>

<div class="box"></div>

</body>
</html>
